Don't care about signed/unsigned compare

Summary:
We need to stop these:
https://github.com/facebook/rocksdb/pull/99
https://github.com/facebook/rocksdb/pull/83

Test Plan: no

Reviewers: dhruba, haobo, sdong, ljin, yhchiang

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16905